The present invention relates to a liquid crystal cell and its manufacturing method. In particular, the invention pertains to a method whereby the surfaces of base plates of the ferroelectric liquid crystal cell are treated by rubbing to provide stable molecular orientation.
Heretofore there have been put into practical use TN (Twisted Numatic) type, DSM (Dynamic Scattering Mode) type, and GH (Guest-Host) type liquid crystal cells which employ nematic liquid crystal. Optical switching elements utilizing these liquid crystal cells have found use in a variety of electro-optical applications such as displays, optical communications, printers, and laser-applied field.
The conventional elements or devices employing nematic liquid crystal, however, pose a problem in their response speed which is several milliseconds at the highest, and hence cannot be used in situations requiring fast response operations. For this reason the prior art liquid crystal cells have been limited to applications which do not call for high-speed response.
In recent years an entirely new type of liquid crystal cell has been proposed which utilizes ferroelectric liquid crystal. The ferroelectric liquid crystal cell is fast in response time and is now attracting attention as a most promising liquid crystal cell because it is suitable for high-speed display and many other electro-optical applications. The conventional nematic liquid crystal cells utilize the dielectric anisotropy for their driving. In contrast thereto, the ferroelectric liquid crystal cell utilizes a large torque PsE which is produced by the interaction between an electric field E and spontaneous polarization Ps; this permits high-speed operation of the ferroelectric liquid crystal cell.
The ferroelectric liquid crystal cell has been proposed by, for example, Iwasaki et al (Y. Iwasaki et al, Jpn. J. Appl. Phys., Vol. 18 (1979), No. 12, pp. 2323, hereinafter referred to as literature (1) and Clark et al (Clark et al, Appl. Phys. Lett., 36 (11), 1 June 1980, pp. 899, U.S. Pat. No. 4,367,924 issued on Jan. 11, 1983, hereinafter referred to as literature (2) In the ferroelectric liquid crystal cell, as shown in FIG. 1A, liquid crystal molecules 1 whose long axes are respectively tilted at an angle +.theta. and at angle -.theta. to the helix axis Y of the liquid crystal in a plane parallel to the cell plate surfaces are mixed together. Upon application of a voltage to the liquid crystal, the liquid crystal molecules 1 are all forced to be tilted at the angle -.theta. alone, as depicted in FIG. 1B, and they remain unchanged even after removal of the applied voltage. Reversing the polarity of the applied voltage, the liquid crystal molecules 1 are all forced to be tilted at the angle +.theta. alone, as shown in FIG. 1C, and their tilt direction remains unchanged even after removal of the voltage. Thus, the ferroelectric liquid crystal cell has two enforced molecular orientation directions, and these directions can be reversed by reversing the polarity of the externally applied field; further, it gives rise to variations in birefringence accordingly. The combined use of the ferroelectric liquid crystal cell and polarizers will permit arbitrary transmission and interception of light through utilization of the above phenomenon.
However, in the conventional ferroelectric liquid crystal cells the alignment technique used for their molecular orientation--the most important technique--does not work well. For instance, the ferroelectric liquid crystal cell proposed in the above-mentioned literature 1 is not in the least satisfactory in this respect. The liquid crystal cell of literature 2 performs the molecular orientation by slightly moving one cell plate relative to the other a plurality of times after introducing the liquid crystal into the cell, but such a method is not suited for mass-production and lacks stability and reproducibility of the orientation.
Recently Goodby et al have proposed to achieve a stable molecular orientation by coating each cell plate with a high molecular layer and rubbing its surface in one direction (J. W. Goodby et al, Ferroelectrics, 59, 1984, pp. 137, hereinafter referred to as literature (3). This method ensures stable molecular orientation in one direction; for example, previous rubbing of the plate surface in the direction -.theta. in FIG. 1 will produce stable molecular orientation when the liquid crystal molecules are tilted in that direction, as shown in FIG. 1B. However, the molecular orientation is not so stable when the liquid crystal molecules are tilted in the direction +.theta., as depicted in FIG. 1C. In this case, after removal of the applied voltage, the molecular tilt angle gradually varies and some molecules restore to the initial direction -.theta. shown in FIG. 1B.
As will be understood from the above, the prior art has failed to provide the stable molecular orientation function which is one of important factors for practical applications of the ferroelectric liquid crystal cell.
To sum up, molecular orientation by the application of a magnetic field, which is applicable to the nematic liquid crystal cell, is almost ineffective for the smectic liquid crystal cell having a thickness of less than several .mu.m. The Clark et al method is theoretically possible but is not suited to mass-production and possesses the defect of poor stability. Furthermore, the combination of the high molecular layer coating and the one-direction rubbing creates stable molecular orientation in one direction but is unstable in other directions.
It is therefore an object of the present invention to provide a ferroelectric liquid crystal cell which has two stable molecular orientation directions and hence is suitable for practical applications and mass-production, and a method for the manufacture of such a ferroelectric liquid crystal cell.